The long term objective is to understand, as completely as possible, the host-parasite relationship in cultured cells and in animals infected with Toxoplasma gondii, an obligate intracellular protozoan parasite. The first specific aim is to determine the mechanism by which gamma interferon (INF) inhibits the growth of T. gondii in cultured cells. This mechanism appears to involve the induction of host cell enzymes that degrade tryptophan to kynurenine. The enzymes responsible for this degradation will be assayed and characterized. The role of the INF-induced degradation of tryptophan in the antitoxoplasma effect will be studied by using mutant host cells that cannot be induced to make the degradative enzymes and by using inhibitors of these enzymes. The possibility that tryptophan degradation products are toxic to T. gondii will be examined as will the possibility that trytophan levels in the INF treated host cell's pool are too low to support parasite growth. The second aim is to examine the in vivo effectiveness of murine gamma-INF in mice infected with T. gondii. Experiments will measure protection against a lethal challenge dose, activation of peritoneal macrophages, and the metabolism of radioactive tryptophan. Recovery from toxoplasmosis is probably mediated primarily by cellular immunity. One aspect of this immunity could be the production of lymphokines such as gamma-INF. In the absence of adequate cell mediated immunity, toxoplasmosis is a serious infection in immunosuppressed patients and especially in the victims of AIDS. The other persons at high risk of severe toxoplasmosis are unborn fetuses. Toxoplasma gondii causes severe congenital defects when pregnant women are infected. Some of these infections result from the ingestion of oocysts excreted in the feces of infected cats. The third aim is to produce a safe and effective feline vaccine that can prevent this transmission. The principle of the vaccine is that the critical antigens for feline immunity are expressed on the T. gondii gametes that are produced only in cat intestinal epithelium. The vaccine will consist of a mutant parasite that cannot make one gamete but can make the other. Without both gametes, infectious oocysts cannot be excreted but one gamete may be sufficient to immunize. The fourth aim is to isolate and characterize more mutants of T. gondii and use them in genetic crosses to define linkage groups of the parasite genome. These mutants will also be used to analyze the events of meiosis, which might be a single step process in T. gondii and other related parasites such as those that cause malaria.